As microelectronic technology has progressed, the degree of circuit integration in microchips has increased to the point that complete electronic devices such as cellular telephones, cameras or handheld computers typically comprise a relatively small number of microchips, along with some discrete components (e.g., resistors, capacitors) connected together on a printed wiring board (PWB).
There are two commonly known types of PWB technologies: the older technology in which component leads are soldered in holes drilled through the PWB, and the currently favored technology in which components leads are soldered to pads on the surface of the PWB. The latter is referred to as surface mount technology (SMT).
Highly integrated microchips often require a large number of external connections. External connections to printed wiring boards are made through leads of packages which enclose the microchips. Each connection between a lead and the PWB takes up a certain amount of space that is determined by a lead traverse dimension (corresponding to PWB pad transverse dimension), and a minimum inter-pad spacing. The minimum inter-pad spacing is dictated by the need to avoid inter-pad short circuits caused by solder bridges. The number of external connections required for a given microchip may be so large as to determine a lower bound on the size of the package for the microchip. That is to say, in order to accommodate the number of leads while maintaining lead spacing the chip package must have a certain size. Thus, even though the microchip itself is highly miniaturized, a large package is necessitated, thereby negating, in certain respects the great expense incurred in miniaturizing the microchip.
Circuits typically include a number of discrete components (e.g., resistors, capacitors). In fact, for many circuits, discrete components take up most of the surface area of the PWB. The area taken up by discrete components is, of course, determined by the size of the discrete components and the required inter-pad spacing. As in the case of packaged microchips, the required inter-pad spacing is dictated by the need to avoid solder shorts. Thus, it is generally desirable to be able to reduce the inter-pad spacing.
In handheld devices, such as portable computers, cameras and cellular telephones, it is particularly important to control the space occupied by circuitry so that the overall size of the handheld device is not excessive and/or so that more functionality can be included in the handheld device. For handheld devices, a single PWB may be used to mechanically support and electrically connect a wide variety of parts including microchips, discrete components, electrical connectors, and EMI/RFI shields. (One use of EMI/RFI shields is to shield RF components mounted on cellular telephone printed wiring boards). In the interest of cost reduction, which is a particular concern for mass produced consumer devices such as cellular telephones, it is desirable to secure all components using solder applied in a single soldering procedure. However, the inclusion of disparate types of devices places conflicting demands on the soldering process by which components are secured to the PWB.
In particular, for microchips and discrete components (e.g., capacitors, resistors) which are not stressed it is desirable to be able to reduce the inter-pad spacing as much as possible. In the past in order to reduce the inter-pad spacing, without incurring electrical shorts due to solder bridges the thickness of solder paste applied to the PWB has been reduced.
However, if components such as electrical connectors which are mechanically stressed are to be included on a PWB it is desirable to secure these with high strength solder joints. In the past engineers resorted to through hole mounting in order to secure mechanically stressed components such as electrical connectors. However, in the interest of simplified more cost effective manufacturing it is desirable to use a single SMT soldering process to secure all components. Conventional wisdom dictates that in order to secure mechanically stressed components using SMT a thicker layer of solder paste be applied, however this is at odds with the above mentioned goal of reducing pad size and inter-pad spacing for microchips and discrete components.
RFI/EMI shields present their own challenges to the solder process. A common type of RFI/EMI shield that is used in handheld devices including cellular telephones is die formed from a piece of flat metal stock into a rectangular or irregularly shaped shallow tub. The rim of the tub is soldered to a congruently shaped closed curve trace on the PWB. For the shield to function effectively it is important that a continuous solder joint be formed all around between the rim and the trace on the PWB. In real world production there is often some appreciable tolerance on the flatness (planarity) of the rim of the shield. Addressing the imperfections in the flatness of the shield calls for using a thicker solder paste, however this is again in conflict with the desire to reduce the inter-pad spacing for the microchips and discrete components.
For large chip packages with multiple leads there may also be a substantial tolerance on the coplanarity of the many leads. Addressing the coplanarity tolerance also calls for using a thicker solder paste, in conflict with the desire to reduce inter-pad spacing.
What is needed is a SMT solder process that is able to achieve reduced inter-pad spacing for microchips and discrete components, is able to secure large components that are mechanically stressed such as electrical connectors, is also able form continuous solder joints to imperfectly flat components, and do so all on a single board with a single soldering procedure.